1. Field of the Invention
The invention relates to a method for filtering a digital value with improved noise behavior and a circuit configuration for carrying out the method.
One such method or configuration is known, for instance, from a paper entitled: "A 16-Bit 4th Order Noise-Shaping D/A Converter" by Carley and Kenney, in IEEE 1988 Custom Integrated Circuits Conference, pp. 21.7.1 ff. Such a system is especially shown in FIG. 1 on page 21.7.1.
In delta-sigma modulators, when digital zero values are input at the output of a digital/analog converter, a noise signal appears that has variously strongly pronounced harmonic components, which are created by a preceding noise-shaping filter that is used. The IEEE Journal of Solid-State Circuits, Vol. 23, No. 6, December 1988, pp. 1351 ff. and IEEE 1990 CH 2868-8/90/0000 pp. 895 ff. and the paper cited above all disclose various methods for improving the signal-to-noise ratio in such devices. However, those methods are relatively complicated and expensive.
Second order noise shapers of that kind, as a function of their internal condition (i.e., memory contents), produce limit cycles, as a result of which harmonic components are created, and can be reinforced, in the noise signal of a following D/A converter.